The invention is related generally to mobile communications and more particularly to a system and method for operating in an area and in a frequency spectrum in which existing communication systems are already operating.
The mobile communications industry has grown at a fast pace in the United States and even faster in some other countries. It has become an important service of substantial utility and because of the growth rate, saturation of the existing service is of concern. Contributing to this concern is the congestion of the electromagnetic frequency spectrum which is becoming increasingly severe as the communications needs of society expand. This congestion is caused not only by fixed microwave service or site (FMS) installations but also by other communications systems including mobile communications systems. In the mobile communications industry alone, it is estimated that the number of world-wide mobile subscribers will increase by an order of magnitude in the next ten years. The radio frequency spectrum is limited and in view of this increasing demand for its use, means to use it more efficiently are constantly being explored.
Increasing demands for mobile radio service have put great strain on the frequency allocation process. This is particularly true for those frequencies currently felt to be uniquely usable for mobile radio; that is, the UHF band from roughly 300 to 3,000 Mhz. Much of this precious spectrum is presently allocated to fixed microwave service. In recognition of the particular value of these frequencies for mobile radio service, the Federal Communications Commission (FCC) has proposed plans for relocating some of the fixed microwave service users to other frequency bands to make room for emerging technologies and personal communications systems particularly in the mobile services. Relocation may be a lengthy process taking place over many years. In the meantime, new mobile services may be licensed on a "not-to-interfere" and "not-to-claim interference" basis with respect to the incumbents.
Detailed study of the geographic distribution of the present fixed microwave service licensees indicates that while in certain geographical areas these bands are almost fully allocated for use as fixed microwave services of a given type, there is still considerable usable frequency spectrum available in the geographical and frequency spaces between the interference zones of present fixed microwave service installations.
As used herein, an "interference zone" refers to a geographic region surrounding a system operating in the electromagnetic spectrum within which harmful interference would be encountered. Should the transmission by a second system interfere with the reception of a first system, the second system would be considered to be operating in the interference zone of the first system. Should the receiver of a second system receive harmful interference from the transmission of a first system, the second system would also be considered to be operating in the interference zone of the first system.
In a system that provides both transmission and reception, there are thus two interference zones and these two interference zones may differ. A "transmit interference zone" is a minimal geographical region about the fixed system's radio communication receiver system defined for a particular frequency band where transmission by a mobile transmitter system on that frequency band in that geographical region could cause harmful interference to the fixed system's receiver system. A "receive interference zone" is a minimal geographical region about a fixed system's radio communication transmitting system defined for a particular frequency band where a mobile receiver system in that frequency band within that geographical region could receive harmful interference from the fixed system's transmitting system. The interference zone is specified by frequency bands and by three dimensions of geography.
Geosynchronous communications satellites over the United States have shared frequency spectrum with fixed microwave service ground towers from the beginning of the commercial space age. Such coexistence is practical because the satellite-to-terrestrial microwave geometry is fixed at a range of about 22,000 nautical miles and at elevation angles greater than about 20 degrees. This provides an acceptably large minimum angle between a geostationary satellite and the fixed ground microwave system. When the ground fixed microwave system follows category "A" antenna requirements in accordance with FCC regulations, and the satellite appropriately limits its flux density on the ground, the two systems can operate on a mutually non-interfering basis. Even though the two systems use common frequencies, the satellite service is not in the interference zone of the ground tower systems and vice versa.
This compatible operation of geostationary satellite systems with terrestrial microwave stations in a common frequency band is facilitated by their fixed positions, the large distance between them, and the directivity of their antennas. Because the satellite system does not move relative to the FMS, it will never be in a geographically interfering position with the terrestrial microwave stations. Such is not the case with mobile users. Because mobile users are capable of continuously changing position and their antennas are not directive, their operation could interfere with an existing station. This arrangement would not provide a suitable "not-to-interfere" system with the existing licensees. However, as discussed above, there exists a large amount of frequency spectrum that is unused between the interference zones of the existing stations.
Hence those skilled in the art have recognized the need for a suitably agile, intelligent communications system having mobile radio users that can coexist with service installations operating in fully allocated frequency bands. Those skilled in the art have also recognized the need for a communications system for mobile users that can coexist on a not-to-interfere and not-to-claim interference basis with fixed microwave service. The present invention meets this need and others.